This invention relates to an improved method for preparing organometallic complexes of magnesium and alkali metals and to novel organometallic complexes of magnesium and alkali metals which are characterized by being soluble in liquid aliphatic or cycloaliphatic solvents in the absence of aromatic solvents or Lewis bases.
This invention is particularly concerned with the production of alkali metal alkyl-dialkylmagnesium compositions. Compositions of this class have been often referred to in the prior art as alkali metal alkyl-dialkylmagnesium complexes, and have been represented by the formula EQU (MR).sub.x.(MgR'R")y
where M is lithium, sodium or potassium, and R,R' and R" are hydrocarbyl groups.
The compositions can also be described as magnesiate compositions, or more specifically alkali metal trialkylmagnesiates, and are represented by the formula EQU M.sub.x Mg.sub.y R.sub.a R'.sub.b R".sub.c
where M, R,R' and R" are as noted above, and x+y=a+b+c. We prefer to use the latter terminology in describing the novel compositions and process of the present invention.
Alkali metal trialkylmagnesiates of the type to which the present invention pertains are useful for various purposes, and especially as catalysts in various polymerization or telomerization reactions. For example, commonly owned U.S. Pat. No. 3,822,219 describes the use of hydrocarbon-soluble complexes of dialkylmagnesium compounds with alkyllithium compounds as initiators in the polymerization of dienes and with alkyllithium or alkylpotassium compounds as catalysts in the telomerization of conjugated dienes, such as 1,3 butadiene, to produce polymers or telomers of predictable microstructure.
Various processes have been employed for producing the alkali metal alkyl-dialkylmagnesium complexes. Malpass and Eastham, J. Org. Chem., Vol. 38, No. 21, 1973, have described a method of preparing organometallic complexes involving magnesium and alkali metals in which alkali metal is used directly to reduce a dialkylmagnesium compound to produce a dialkylmagnesium-alkali metal complex. However, in the process, part of the dialkylmagnesium is reduced to magnesium metal. Consequently, the resulting yields based upon the initial magnesium are relatively low, and the ratio of alkali metal to magnesium in the final product is less than that which would be desired. Also, in practice, it has been found that the presence of an aromatic solvent, such as benzene or toluene was necessary to maintain good solubility.
The aforementioned U.S. Pat. No., 3,822,219, describes a process illustrated by the production of a complex of a hydrocarbon soluble mixed primary alkyl-, secondary or tertiary alkylmagnesium with an alkylsodium or alkylpotassium. First a primary, linear dialkylmagnesium compound is prepared directly in a hydrocarbon solvent from magnesium metal and the corresponding alkyl halide. Then a secondary or tertiary alkyllithium is added to react the by-product magnesium halide formed in the direct preparation step. The resulting liquid solution is separated from the solid lithium chloride and the liquid solution containing a complex of a primary dialkylmagnesium with a secondary or tertiary dialkylmagnesium is then complexed with a Group I metal organic compound such as n-butylsodium.
This method requires the use of solid alkali metal alkyls, such as n-butylsodium, which are difficult to handle, and the yields obtained are somewhat lower than would be desired. Also, in all of the examples of this patent, aromatic hydrocarbons such as benzene are used to maintain the solubility of the composition in the solvent. The presence of aromatic hydrocarbons or Lewis bases (such as tertiary amines and ethers) in the complex is undesirable in certain applications, such as in anionic polymerizations, because they serve to act as chain transfer agents and/or act to decompose organometallic initiators such as alkyllithium or alkylsodium. In addition, they may require subsequent removal from the polymerization solvent.
With the foregoing in mind, it is an object of the present invention to provide an improved process for producing an alkali metal trialkylmagnesiate which provides improved yields.
It is a further object of the invention to provide an improved process as noted above which avoids loss of the expensive dialkylmagnesium reactants by reduction to magnesium metal.
Still another object of the present invention is to provide an alkali metal alkyl-dialkylmagnesium complex which is soluble in a liquid aliphatic or cycloaliphatic solvent in the absence of aromatic solvents or Lewis bases.